This invention relates generally to a plant for the combustion of fossil fuel and biomaterial or waste. More particularly, the present invention relates to a plant in which fossil fuel is combusted in a large boiler plant of a power plant, biomaterial or waste is combusted in an additional combustion device configured as a forehearth to the large boiler plant, and flue gas arising in the additional combustion device is introduced into the large boiler plant.
In addition to waste, which may contain organic and inorganic waste materials, only organic waste materials or biomaterials (also denoted as biomass), such as wood wastes of all types, bagasse, peanut and other shells or secondary biomaterials such as reeds, straw, underbrush, etc. can be combusted in such forehearths. On the one hand, fossil fuel can be substituted by the biomaterial by burning such biomaterials in forehearths of large boiler plants operating with fossil fuel, and thus CO.sub.2 emission will be reduced, and, on the other hand, valuable landfill space will be spared.
DE 4,442,136 A1 discloses a power plant designed for burning fossil fuel having a water/steam circuit for decoupling of heat, a large boiler plant, and an additional combustion device, designed for combustion of waste, which acts as a forehearth to the large boiler plant. A grate hearth is utilized as the additional combustion device. The flue gas arising in the additional combustion device is conducted into the large boiler plant, whereby the flue gas leaving the large boiler plant derives from a maximum of 30% from the grate hearth or the forehearth firing and a minimum of 70% from the combustion of fossil fuel. This device may utilize nearly unprocessed waste while assuring that the legally valid conditions for the flue gas are maintained. This applies particularly to maintaining HCl and CO limiting values and the burning off of possible other pyrolysis products.
DE 4,312,900 A1 discloses a coal dust-fired steam boiler, in which the biomaterials are fed into the forehearth for burning, and the hot gases produced thereby are conveyed to the combustion chamber of the steam boiler. The forehearth is operated directly with the system of hot air, pilot burner, pulverizer, principal burner, combustion chamber, heating surfaces, flue gas and ash removal, as well as with the safety and emergency connections of the steam boiler. The patent also discloses that heating surfaces can be arranged in the forehearth, which are integrated in the heating-surface system of the steam boiler or in a heat-removal system.
For the operation of such a plant for combustion of fossil fuel and biomaterial or waste however, considerable effort must be expended to obtain an optimal efficiency of the plant, to achieve operating costs that are as small as possible for preparing the biomaterial or waste "fuel", to minimize investment costs, and to obtain a smaller space requirement for the plant. All previously known measures represent only a partial solution of an individual problem and do not offer a complete solution for all of these problems.